This invention relates to pumps having magnetically suspended rotors with three translational and three rotational axes of motion, one of which (translational or rotational) is actively controlled, another of which (rotational) is actively driven, and the remainder of which (translational and rotational) are passively controlled (meaning, for example, that no electronic controller is required). Such pumps are particularly suited to the task of pumping blood in humans and other animals.
There are many types of fluid pumps suitable for use in a wide range of applications, all performing the same basic function of moving fluid from one point to another, or moving a fluid from one energy level to another. However, pumps for pumping sensitive fluids, such as blood, introduce special design requirements. Additionally, pumps for implantation in a human patient for long or short-term use as ventricular assist devices (VAD""s) or complete heart replacement, add additional size, weight, durability, and other requirements.
The design problems associated with sensitive fluids, including blood, generally relate to problems caused by contact of the fluid with mechanical parts and other substances present in the pump. Problem contact areas for sensitive fluids may include contact with materials and structures in rotating fluid seals, contact with mechanical bearing assemblies that are exposed to the fluid, and use in bearing structures that depend on a layer of fluid between moving, surfaces to provide reduced friction, such as hydrodynamic bearings. For example, it is well known that rotating shaft seals are notoriously susceptible to wear, failure, and even attack by some fluids. Many types of pumps may also increase mechanical working of the fluid and precipitate detrimental processes such as chemical reactions or blood clotting.
It is also well known that pumps for corrosive fluids, blood, and fluids used in food processing require careful design of the flow passages to avoid fluid damage, contamination, and other undesirable conditions. For example, ball bearing and other rolling-element bearings must in general be used with some type of shaft seal to isolate the fluid from the bearing. This may be needed to prevent damage to the bearing by caustic fluids, or to prevent damage to the fluid by the rolling elements of the bearing. For example, rolling-element bearings can crush and destroy the living cells in blood. Thus, rolling-element bearings are generally not practical for blood pumps.
Moreover, high shear and stagnation should be avoided in blood pumps. It is well known that there are limits to the time that red blood cells can withstand high mechanical shear. Red blood cells are subject to damage or rupture (hemolysis) if these limits are exceeded. In the other extreme, blood is particularly susceptible to clotting in regions of stagnation and low flow.
Finally, the size, weight, biocompatibility, and operating durability and reliability of blood pumps are a major concern when such pumps are used as VAD""s or heart replacement pumps. It would be desirable to have a VAD or heart replacement pump that can operate reliably for periods of time up to twenty or thirty years despite the normal bumping and jarring of everyday life, including unexpected impact such as from falling, yet is small enough to implant easily in a patient""s chest. It is also desirable to reduce the power requirements of such a pump so as to minimize battery size and thus increase mobility of the patient.
To address these problems, pumps with magnetically suspended impellers have been developed. For example U.S. Pat. No. 5,112,202 to Oshima discloses a pump in which the impeller is magnetically suspended or levitated within the pump housing, and is magnetically, not mechanically, coupled to the pump housing. The pump employs permanent magnets rotating, on a motor external to the pumping chamber, with the external permanent magnets magnetically coupled to opposing permanent magnets on the impeller. Such magnetically suspended pumps are well adapted to pumping sensitive fluids because they eliminate the mechanical bearing structure or rotating seals which can damage, or be damaged by, the fluid.
However, such pumps also present several drawbacks. First, an external motor with its own means of bearing support (ball bearings) is still required to rotate the impeller. It is the external bearing support that maintains the position of the rotor in such a pump. Though the motor is sealed from contact with blood and other bodily fluids, and is magnetically coupled to the suspended impeller, it still employs bearings that produce heat and can be prone to failure. Naturally, such pumps tend to be bulky in part because of the size of the electric motor. These pumps are frequently unsuitable for implantation in a patient because of size, weight, power consumption, and durability problems.
Other methods of magnetically supporting a rotating pump impeller have been developed. For example, U.S. Pat. No. 4,688,998 to Olsen teaches a fully suspended pump rotor employing permanent magnet rings on the rotor magnetized along the axis of rotation, and actively controlled electromagnets on the stator that create a magnetic field to stabilize the position of the rotor. This approach also leaves certain problems unsolved. While the manufacture of permanent magnets has advanced substantially, there are still significant process variations. These variations include repeatability from one magnet to the next, and homogeneity of the material within one magnet. The position and stability of the rotor in the Olsen invention is entirely dependent on the homogeneity of the permanent magnet rings. These problems are well known by designers of electro-mechanical devices, where significant steps are normally taken to reduce the dependency of device performance on homogeneous magnets. In the field of permanent magnet motors, this is a well known source of torque ripple.
U.S. Pat. No. 5,443,503 to Yamane discloses an artificial heart pump that includes a cylindrical stator surrounding and magnetically suspending a rotor that contacts the pump""s housing at a mechanical pivot point. The failure to provide a contactless rotor increases the heat generation and energy consumption of the rotor, thus making this pump less than desirable as a blood pump. In addition, the mechanical pivot point is a location of blood damage and stagnation that may lead to clotting.
There is thus an ongoing interest in providing a practical implantable blood pump with a magnetically suspended, contactless rotor for pumping human blood without damaging the blood. Such a pump should have reduced complexity, reduced cost, and improved reliability.
An inventive blood pump in accordance with this invention includes a housing that has inlet and outlet ports for receiving and discharging blood. A rotor is positioned in the housing""s interior for pumping blood between the housing""s inlet and outlet ports, with the rotor being capable of motion in three translational and three rotational axes. An assembly for magnetically suspending and rotating the rotor in a contact-free manner with respect to the housing includes only one electromagnetic bearing that actively controls motion of the rotor with respect to one axis selected from the rotor""s three translational and three rotational axes, an electromagnetic motor that actively drives motion of the rotor with respect to one of its three rotational axes, and magnetic bearings for passively controlling motion of the rotor with respect to the remaining four of its translational and rotational axes. As used herein, the term xe2x80x9cpassively controllingxe2x80x9d refers, for example, to a method of controlling motion of the rotor using magnetic fields that does not depend upon an electronic controller to modulate the fields. Also, the inventive blood pump can be incorporated into an artificial heart or ventricular assist device.
In another embodiment of this invention, blood is pumped through a human or other animal""s body by immersing a pump rotor capable of motion in three translational and three rotational axes in the blood. The pump rotor is magnetically suspended in a contact-free manner with a plurality of passive magnetic bearings and only one active electromagnetic bearing structure, and the pump rotor is rotated with a magnetic motor. The magnetic suspension and rotation of the pump rotor is actively controlled with respect to only one of the pump rotor""s three translational and three rotational axes using the electromagnetic bearing structure.